1. Field of the Invention
The present invention relates to packet transfer methods and node apparatuses, and more specifically, to a packet transfer method and a node apparatus that can suppress an increase in unnecessary traffic when the route is looped because of incorrect routing information or the like.
2. Description of the Related Art
As indicated in RFC 791, a time-to-live (TTL) field is provided in an Internet Protocol (IP) header, so that infinite packet transfer can be avoided even if the route is looped because of a mismatch occurring in routing information.
More specifically, a source node from which an IP packet is sent sets the TTL value to a relatively large positive value (‘64,’ for example) when it sends the packet, and a node (a router or the like) that transfers the IP packet decreases the value of the TTL field in the IP header by ⊥ each time the packet is transferred. An IP transfer function is defined not to transfer but to discard a packet if its TTL value reaches ‘0.’
The loop prevention method using TTL is one of the basic functions of the IP, and technologies such as the one disclosed in Japanese Unexamined Patent Application Publication No. 2004-56293 are provided to improve the effectiveness of the method. In the technology disclosed in this patent document, for example, the entry corresponding to a packet discarded because the TTL value had reached ‘0’ is marked in a transfer table, and the entry indicates that the packet was discarded.
Recently, the multiprotocol label switching (MPLS) technology and the generalized MPLS (GMPLS) technology are widely used to form a virtual private network (VPN) on a network and to implement multi-layered routing control or quality of service (QoS) control. As indicated in RFC 3443, how to deal with the TTL when the MPLS is used is also specified, and loop prevention is implemented even through a tunnel.